update clap to 3.0

Currently the only RUSTSEC affecting arti is RUSTSEC-2021-0139 for ansi_term. It's about ansi_term being unmaintained. This is not a vulnerability per se, but it would still be nice to not have any exception at all.

clap 2.x is what pulls ansi_term, and its v3 does not depend on ansi_term. We could get ride of the exception by upgrading to clap 3.x. I don't know if this is a smooth update or not.